1. Field of the Invention
This invention relates to display systems, more particularly to displays systems that generate images using colored filters.
2. Background of the Invention
One of the most common display devices is the cathode-ray tube (CRT), which generates colors from phosphors. The phosphors luminesce when struck by a stream of energy from the cathode-ray tube. The color that is produced depends upon the frequency of the energy.
A class of newer display systems do not use CRTs, but create images using an x-y grid of individually controllable elements. These devices, spatial light modulators, typically have one or more elements on their grid that correspond to a picture element (pixel) of the final image. They typically create color images by changing the color of the light that strikes the element, or the light transferred by the element to the display surface. The appropriate combination of colors, typically red, green and blue, and amounts of each color are determined for each pixel. The elements are then controlled to produce the proper amount of each color during a display frame time to allow the eye to integrate that mix of colors into the proper color.
The colored light can be produced in several ways. The system could used three separate devices, each with their own light sources of the appropriate colors. Alternately, the system could use three devices, but have one light source, with beam splitters splitting the appropriate light color prior to striking the device for that color. This last embodiment is more common, since having to use three light sources raise the cost of the system too much to make it practical. A system using three devices will be referred to as a spatial coloring system.
Use of one device and light source, or two devices and either one or two light sources are also possible. These systems can be more desirable, since having fewer devices means less cost. However, in order to produce two or three colors during one frame time requires some type of time division of the frame among the colors. Using two devices means that one device will have to produce two colors, and one device systems have to produce all three colors on that device. The time allocated for any color must be shortened. This type of filtering system will be referred to as a temporal coloring system.
Using one light source, whether it be for a spatial or temporal coloring system, reduces the available light for each color. For three-device systems, each device gets 33% (1/3) the light from the source, as does a one device system. Systems with two devices have one device that can get up to 50% of the light for the device processing one color, and as little as 25% of the light for the device handling two colors.
One solution for this problem has been suggested in U.S. Pat. No. 5,233,385, titled "White Light Enhanced Color Field Sequential Projection," and assigned to Texas Instruments, Incorporated. In that patent, the system added white light as either a segment of a color wheel, for temporal coloring systems, or a fourth device, for spatial coloring systems. However, the white portion of the frame was merely to add an overall brightness "floor" to the image. Depending upon the characteristics of the light source, the filters used, and the image being projected, this result can cause a washing out of the colors, especially in the high brightness areas of the image. Too much white light added to any of the primary colors causes that primary to become almost a pastel. However, the need to add brightness to the overall picture remains a problem.
Therefore, a solution is needed that allows addition of white light to increase overall brightness but that controls the white light to prevent washing out of the colors.